Age-related changes in levels, actions, and secretion of the somatomedins and related growth-promiting hormones are being studied. Fischer 344 rats from the NIA barrier-protected colony at Charles River are used for all aging studies. Serum content of active somatomedin is measured by a cell growth bioassay and this is confirmed with a receptor displacement assay. Secretion of sommatomedins by hepatocytes from rats of various ages will be investigated.